With the advent of semi-conductor technology, current circuit design requires utilization of many types of smaller and smaller circuit components which are assembled on printed circuit boards or substrates. Numerous miniature circuit elements and combinations thereof have been encased in small machine insertable packages known as DIPS (Dual In-Line Packages). Not only is there a need for packaged active circuit elements but also a need for passive elements, such as capacitors and resistors. One type of miniature capacitor that lends itself to packaging for machine insertion is the ceramic capacitor. However, ceramic capacitors are subject to several deterrents, such as temperature sensitivity, non-self healing, capacitor degradation over a period of time, and the character of failing as a short circuit upon breakdown. A more reliable capacitor not subject to the aforementioned deterrents is the metallized film rolled capacitor. Use of rolled film capacitors has been curtailed because of their relatively large size, cost and unadaptability to miniature packaging for machine insertion.
The present invention is concerned with a machine for producing miniature metallized film rolled capacitors which are very small, inexpensive and suitable for packaging into small cases that may be readily machine inserted in a printed circuit board.
Rolled metallized film capacitors are usually manufactured by masking a plastic film being advanced through a vacuum metallizer which deposits a thin layer of metal, such as zinc or aluminum, through the mask onto one exposed surface of the film to form stripes of non-metallized areas interposed between longitudinally extending metallized areas. The metallized film is cut along the middle of the unmetallized stripes and the middle of the metallized areas to provide pairs of metallized film strips each having a non-metallized margin. A pair of films with opposed unmetallized margins are wound together with the metallized surface of each film abutting the unmetallized surface of the other film. The winding is such that there is a misregistration of the margins so that the metallized edges of the wound film strips extend slightly beyond the unmetallized edges. Near the completion of the winding operation, a strip of non-coated dielectric material is inserted and wound between the peripheral windings of the metallized strips. The inserted strip permits the heat sealing of the outer convolutions to preclude unraveling without short circuiting the metallized portions of the wound film. These rolled capacitor blanks are metal sprayed with heat fusible metal on opposite ends and then wax impregnated for moisture protection. Suitable electrical terminals are then attached to the metal sprayed ends.
During the metallization of the film, certain portions of the metal have a tendency to work under the mask so that there is no finite line of demarcation between the metallized and unmetallized stripes. In winding a pair of such films, an added amount of width of unmetallized margin area is provided to insure that any metal projecting into the non-metallized margin does not short circuit with the metal on the other film. Further, during the various prior art fabricating steps, the films are wound and rewound resulting in a weave lay or lateral overlay of adjacent convolutions which necessitates an added amount of misregistration in order to obtain the desired amount of metallized surface area overlap to form the capacitor plates. This added misregistration results in a relatively long capacitor blank.
Currently, the usual type of winding machine winds a succession of capacitor blanks on a pair of winding mandrels mounted in a turret. The turret is rotated to move the mandrels successively into a winding position where the mandrel is rotated to wind a pair of films by drawing the films from a pair of supply rolls. The metallized films must be of sufficient thickness and strength so as to preclude breaking upon impartation of the mandrel winding forces. The use of relatively thick films results in the production of a capacitor blank having a relatively large diameter.
One type of winding facility for making metallized film resistor-capacitor networks is shown in U.S. Pat. No. 3,939,440 to R. D. Berg et al. In this patent a pair of metallized films with opposed unmetallized margins are simultaneously wound on a commercial capacitor winder identified as Model SAM 2901A capacitor winder obtainable from E. W. Barton Co., San Fernando, Calif. In this patent there is described a method of laser scribing the metallized surface to form resistive paths that in conjunction with the non-scribed metallized surfaces form a resistance capacitance network.